1. Field of the Invention
The present invention relates to a projection exposure apparatus for use to manufacture semiconductor devices and the like.
2. Related Background Art
Recently, a step-and-repeat type contraction projection exposure apparatus (a stepper) has been, as an apparatus for transferring micropatterns at a high resolution, widely used in the lithography process of the semiconductor device manufacturing process In a stepper of the aforesaid type, the exposure and the transference operations are performed by using a projection optical system in such a manner that the patterns of masks or reticles (hereinafter called "reticles") are sequentially stacked on a plurality of shot regions (regions in which circuit patterns will be formed) disposed on a photosensitive substrate (semiconductor wafer sheets or glass plates having the surfaces on each of which a resist layer is formed) in the matrix configuration.
Therefore, the projected image of the reticle pattern must be accurately superposed (aligned) on the shot region. Hence, a variety of systems adaptable to the locating device (an alignment system) for accurately aligning the reticle patten and the wafer (the shot region) to each other have been disclosed as classified into two major types; an on access system and an off-access system.
The off-access system is a system arranged in such a manner that an alignment mark provided in the vicinity of the shot region is limitedly detected by the alignment system and then the wafer (a stage) is moved from the position of the alignment mark for a predetermined distance (the base line quantity) so that the reticle pattern is superposed on the shot region before exposure to light is) performed.
That is, the position at which the mark is detected by the alignment system and the position at which the reticle pattern is exposed to light by the projection optical system are different from each other.
Therefore, in the off-access system, the base line quantity can undesirably be varied due to heat or vibrations, causing a problem to arise in that the accurate superposition cannot be easily realized and, hence, the base line must be frequently measured, for example, whenever the wafer is exchanged in order to improve the superposition (alignment) accuracy.
On the contrary, the on-access system reveals an advantage in that the exposure can be performed in an accurately aligned state because the aforesaid mark detection position and the exposure position are the same.
As the alignment system adaptable to the on-access system, a so-called TTR (Through The Reticle) type alignment system has been available which is so arranged that an alignment mark (hereinafter called a "reticle mark") disposed in the vicinity of the reticle pattern and an alignment mark (wafer mark) on the wafer are detected by means of the projection optical system.
The disclosed alignment systems of the TTR type are mainly classified into the following two types:
One alignment system is arranged in such a manner that alignment irradiation light having substantially the same wavelength region as that of exposure light (i-rays, KrF eximar laser beams or the like) is used to irradiate the reticle mark and the wafer mark and the light reflected from the two marks are detected so that positions of the reticle and the wafer are aligned. In usual, the image of the reticle mark and that of the wafer mark are simultaneously detected by an image pickup device (such as a CCD camera) so that the relative deviation between the two marks is obtained.
Another alignment system is arranged in such a manner that alignment light (for example, He-Ne laser or the like) having a wavelength region which is different from that of exposure light is used. Since the chromatic aberration of the projection optical system can be generally satisfactorily corrected with respect to the exposure wavelength, the reticle and the wafer are aligned to each other by disposing a chromatic aberration correction optical system at a position between the reticle and the wafer.
FIG. 21 illustrates an example of the conventional projection type exposure device for performing the alignment operation by the TTR method. Referring to FIG. 21, a wafer 2 is placed on a wafer stage 1, and an alignment mark 3 is formed in the periphery of each shot region on the wafer 2. A pupil portion of a two-side telecentric projection optical system 4 secured above the wafer stage 1 has a diaphragm 5 fastened thereto, the diaphragm being disposed for the purpose of limiting the NA (Numerical Aperture) of projecting light with which the wafer 2 is irradiated. The wafer stage 1 is constituted in such a manner that it can be freely moved in parallel and rotated by a predetermined angular degree in a plane perpendicular to the optical axis of the projection optical system 4. The position of the wafer stage 1 is detected by a laser interference measuring machine comprising a movable mirror 6, which is disposed on the stage 1, and a detector 7 (although omitted from illustration two pairs each of which is composed of one movable mirror 6 and one detector 7 are provided).
A drive mechanism 8 moves and rotates the wafer stage 1. When rough alignment is performed, a control device 9 causes the drive mechanism 8 to move the wafer stage 1 so as to align the position detected by the laser interference measuring machine to a predetermined position. When a fine alignment is finally performed, an image signal processing circuit 10 supplies information about the position deviation of an alignment mark to be described later to the control device 9 so that the control device 9 locates the wafer stage 1 in such a manner that the aforesaid position deviation is limited in a predetermined range.
An exposure lighting system 11 generates irradiation light IL1 which has a wavelength region (for example, g-rays the wavelength of which is 436 nm, i-rays the wavelength of which is 365 nm or eximar laser beam the wavelength of which is 248 nm) which is intensely sensitive to a photosensitive material such as a photoresist present on the wafer 2. Illumination light IL1 is condensed by a condenser lens 12 so that the pattern region of a reticle 14 secured to a support member 13 is irradiated with condensed irradiation light. Light, which has passed through the reticle 14, is used to irradiate the wafer 2 by the projection optical system 4, so that the image of the pattern region of the reticle 14 is formed on the wafer 2.
Then, alignment is so performed that the deviation between the position of the image of the reticle 15 formed in the vicinity of the pattern region of the reticle 14 and that of the wafer mark 3 placed on the wafer 2 is included in a predetermined range.
A resist layer is formed on the surface of the wafer 2 in the exposure process and the wafer mark 3 is detected via the resist layer at the time of the alignment process. As the resist layer, use of a multilayer resist structure or the like capable of raising the absorption factor (lowering the transmission factor) with respect to exposure light has been considered in order to form a high resolution pattern. However, if the multilayer resist structure or the like is used, the use of light in a wavelength region, which is the same as that of exposure light, as irradiation light for alignment will raise a problem as follows: irradiation light for alignment can be excessively decayed before it reaches the wafer mark 3 on the wafer 2 and also light (regular reflection light, scattered light, diffracted light, and the like) reflected by the mark 3 is decayed, causing the problem to take place in that the mark 3 on the wafer 2 cannot be recognized by the alignment optical system with a sufficient light quantity.
It might therefore be considered feasible to employ light in a wavelength region which has a high transmission factor with respect to the photosensitive material such as the resist, for example, light in a wavelength region which is longer than the wavelength region of exposure light as irradiation light for alignment. Since the chromatic aberration of the projection optical system 4 is, at present, satisfactorily corrected with respect to only irradiation light for exposure in usual, the mark 15 on the reticle 14 and the mark 3 on the wafer 2 do not have a conjugate relationship with respect to the projection optical system 4 in a state where they are irradiated with irradiation light for exposure. Accordingly, a correction optical system for correcting the chromatic aberration is hitherto provided for the purpose of correcting the chromatic aberration of the projection optical system 4 with respect to alignment light so as to make the mark 3 and the mark 15 hold the conjugate relationship.
Then, an alignment optical system having a correction optical system of the aforesaid type will now be described. Referring to FIG. 21, irradiation light IL2 for alignment is supplied through an optical fiber bundle 16, illuminating light IL2 being then converted into substantially parallel light beams by a collimeter lens 17. The parallel light beams are converged on the mark 15 placed on the reticle 14 by a half mirror 18, an objective lens 19 and a mirror 20. Irradiation light, which has passed through the mark 15, is converged onto the mark 3 on the wafer 2 by a correction lens 21 for correcting the chromatic aberration and the projection optical system 4.
Light reflected from the mark 15 on the reticle 14 reversely passes through the optical path while passing the half mirror 18 before it is, by an image-forming lens 22, converged onto the surface on which a charge-coupled type image pickup device (CCD) 23 picks up an image. On the other hand, light reflected from the mark 3 on the wafer 2 is first imaged at a position of the mark 15 on the reticle 14, and then it reversely passes the optical path before it passes through the half mirror 18. Then, it is again imaged on the surface on which the CCD 23 picks up an image by the image-forming lens 22. If the marks 3 and 15 are respectively formed into slit-like patterns, a slit image 15I of the mark 15 on the reticle 14 and an image 31 of the mark 3 on the wafer further formed on the reticle 14 are formed on the surface on which the CCD 23 picks up an image. Information about the aforesaid images is, as picked-up image signal as shown in FIG. 23, supplied to an alignment device 10.
The image signal processing circuit 10 detects the position deviation between the midpoint of the image 15I and that of the image 31 so as to supply information about the deviation to the control device 9. The control device 9 locates the wafer stage 1 so as to cause the aforesaid position deviation to be included in a predetermined range. Incidentally, only one alignment optical system 1 is illustrated in FIG. 21 for the purpose of simplifying the description, four pairs of the alignment optical system, the alignment marks 3 and 15 are present for example. By performing the alignment by means of the alignment optical system, accurate alignment regarding of the parallel movement and rotation of the wafer stage 2 in the plane perpendicular to the optical axis of the projection optical system 4 can be performed.
A system which uses a diffraction grating has been known as a further accurate alignment system (see U.S. Pat. No. 5,004,348 for example). The aforesaid system uses alignment light in a wavelength zone which is different from the wavelength zone of exposure light so as to optically detect one-dimensional diffraction grating mark formed on the wafer or the reticle. In accordance with information of the pitch of the diffraction grating marks, the pitch-directional position of the wafer or the reticle is detected at high resolution. In an interference alignment method of the aforesaid type, a one-dimensional interference fringe is formed by simultaneously irradiating the diffraction grating mark with substantially parallel laser beams from two directions and refracted light generated from the diffraction grating mark due to the irradiation of the thus-formed interference fringe is photoelectrically detected, so that the position deviation of the diffraction grating mark is detected in accordance with the output signal denoting the result of the detection.
The interference fringe alignment method is classified into a heterodyne method in which the irradiation laser beams to be emitted from the two directions are given a predetermined frequency difference and a homodyne method in which the frequency difference is not given. In the homodyne method, stationary interference fringe is formed in parallel to the diffraction grating and therefore the diffraction grating on the wafer or the like must be slightly moved in the direction of the pitch of the grating at the time of performing the position detection. Hence, the position of the diffraction grating is obtained from the interference fringe for use as the standard. On the other hand, in the heterodyne method, the difference in the frequency (the beat frequency) between the two laser beams will cause the interference fringe to flow at high speed in the direction of its pitch at the beat frequency. Therefore, the position of the diffraction grating is obtained from the phase difference between a photoelectric signal obtained at the beat frequency and a reference signal.
However, the conventional projection-type exposure apparatus shown in FIG. 21 necessitates a fact that the mirror 20 and the correction 21 are inserted at the time of the alignment process and the mirror 20 and the correction lens 21 are moved outside the region through which exposure light passes at the time of the exposure process. Therefore, there arises a problem in that the alignment accuracy gradually deteriorates because the position of the optical system is gradually deviated due to the repetition of the forward/rearward movement of the optical system. What is worse, the aforesaid forward/rearward movement of the optical system takes a considerably long time, causing an excessively long time takes to transfer the pattern of the reticle 14 onto the wafer 2 by the step-and-repeat manner. As a result, a problem arises in that the number (the throughput) of wafer sheets which can be transferred and exposed per unit time decreases.
Referring to FIG. 21, the mark 15 on the reticle 14 and the mark 3 on the wafer 2 hold a conjugate relationship when they are irradiated with exposure light. Therefore, the resist layer covering the mark 3 on the wafer 2 is exposed with irradiation light which has passed through the mark 15 on the reticle 14 at the time of the exposure process, causing another problem to arise in that the mark 3 is broken due to development. Although the mark 3 can be again printed because the breakage of the mark 3 is not a critical problem, a problem arises in that a wide mark region must be formed on the wafer 2. If the mark 15 on the reticle 4 is moved away from the pattern region in order to prevent the aforesaid fact, the mark 3 on the wafer 2 must also be moved away from the pattern region in proportion to the aforesaid movement.
However, in a case where the transference and exposure are repeated by, for example, the step-and-repeat method, the alignment mark 3 is formed in a street line region which is an intermediate region between the adjacent shot regions on the wafer 2. However, if the mark 15 on the reticle 14 is moved away from the pattern region, the street region must have a wide area. In a case where the area of street line region is widened, a problem arises in that the shot configuration density on the wafer 2 (the number of the shot regions decreases) is lowered.
With regard to this, Japanese Patent Laid-Open No. 1-140719 has disclosed a projection exposure apparatus so arranged that a fresnel zone plate which serves as a lens with respect to only alignment light and which does not affect exposure irradiation light is used in place of the correction lens 21 shown in FIG. 21. Also in this case, a beam bender corresponding to the mirror 20 shown in FIG. 21 is used. Therefore, the aforesaid problem that an excessively long time takes to perform the transference and the exposure cannot be overcome because there is a necessity of moving forward/rearward the bender. What is worse, a problem arise in that the image forming characteristics at the time of the exposure somewhat deteriorate because exposure light passes through the fresnel zone plate.
As a projection-type exposure apparatus so arranged that the diffraction grating mark serving as the alignment mark is detected by the interference fringe alignment method, a projection-type exposure apparatus has been disclosed in Ser. No. 624, 534 (filed on Dec. 10, 1990 now abandoned). According to this disclosure, a dichroic mirror and a dual focal point device or the like are used for example so that the alignment can be performed at all times and as well as the irradiation of the alignment mark on the wafer with irradiation light for exposure is prevented. However, since the aforesaid arrangement is also arranged in such a manner that the alignment mark on the reticle and the alignment mark on the wafer hold the substantially conjugate relationship, the alignment mark on the wafer cannot be allowed to satisfactorily come closer to the shot region.
Another method has been suggested which is arranged in such a manner that a lens for correcting the chromatic aberration of the alignment beam is, in place of the fresnel zone plate, added to a local region in the projection optical system so as to simultaneously measure the reticle mark and the wafer mark (for example, see DEEP VV WAFER STEPPER WITH THROUGH THE LENS WAFER TO RETICLE ALIGNMENT, SPIE vol. 1264, Optical/Laser Microlithography 3 (1990)).
In connection with this, an optical member to be disposed in the projection optical system may be a diffraction grating disposed locally in place of the small lens, resulting in a similar effect to be obtained. If the small lens is used, light beams for exposure are refracted as well as the alignment light beams, causing the image forming characteristics to deteriorate. However, the use of the diffraction grating will prevent the deterioration.
If a phase-difference type diffraction grating is used, the diffraction grating can be so formed that only alignment light is diffracted but exposure light having different wavelength from that of alignment light is not diffracted by controlling the phase difference quantity (that is, controlling the thickness of the phase material).
By using an optical member of the type for correcting the chromatic aberration with respect to the light beams for alignment, the reticle mark and the wafer mark can be simultaneously measured and therefore the alignment accuracy of the projection exposure apparatus can be improved.
FIG. 24 illustrates an image forming relationship in a case where the diffraction grating is, as the optical member for correcting the chromatic aberration, disposed in a projection optical system. Referring to FIG. 24, two diffraction gratings 90a and 90b are disposed adjacent to the pupil surface of the projection optical system 4 at positions relatively away from the optical axis. Although FIG. 24 illustrates due to the convenience that the diffraction gratings 90a and 90b are positioned on a plane running parallel to the surface of the drawing sheet, they are disposed on a plane perpendicular to the optical axis of the projection optical system 4.
Exposure light Le emitted from one point of a surface, on which the pattern of the reticle 14 is formed, is converged onto a point (conjugate point) on the wafer 2 by the projection optical system 4. However, 0-order light La0 among alignment light Lai emitted from one point on the surface, on which the pattern of the reticle 14 is formed, which as it is passes through the diffraction gratings 90a and 90b is converged to, for example, a point on a plane below the wafer 2 due to the chromatic aberration of the projection optical system 4. In other words, if the diffraction gratings 90a and 90b are not present, the image of the wafer mark of the wafer 2 is not formed by the projection optical system 4 under alignment light on the surface on which the pattern of the reticle 14 is formed. Therefore, in a case where observation systems (which correspond to elements 19, 22 and 23 shown in FIG. 21) for observing the reticle mark and the wafer mark are present above the reticle 14, the image of the reticle mark and that of the wafer mark cannot be simultaneously observed on a common image pickup surface by using common alignment light.
In another case where the diffraction gratings 90a and 90b are disposed, 1-order diffraction light Lal due to the diffraction grating 90a and 90b among alignment light Lai emitted from one point of the reticle 14 is converged to a point (a conjugate point under exposure light) on the wafer 2. On the contrary, 1-order diffraction light due to the diffraction gratings 90a and 90b among alignment light reflected from one point on the wafer 2 is converged to a point on the reticle 14. That is, the presence of the diffraction gratings 90a and 90b causes the surface on which the pattern of the reticle 14 will be formed and the exposure surface of the wafer 2 to hold the conjugate relationship. If a predetermined phase gratings are used as the diffraction gratings 90a and 90b only the intensity of 1-order light Lal can be raised while making the intensity of 0-order light to be substantially zero. Therefore, the loss of alignment light can be reduced to a quantity which can be substantially ignored.
Hence, in the case where the diffraction gratings 90a and 90b are present, the image of the reticle mark formed by alignment light reflected by the reticle mark on a surface of the reticle 14 on which the pattern is formed and the image of the wafer mark due to alignment light reflected from the wafer mark after it has passed through the reticle mark and is used to irradiate the wafer mark on the wafer 21 by the projection optical system 4 are formed on the same image pickup surface in an observation system above the reticle 14. Therefore, the positional relationship between the reticle mark and the wafer mark can be simultaneously and accurately measured.
However, in a case where the optical member for correcting the chromatic aberration is used as described above, a problem arises in that the optical members (the diffraction gratings 90a and 90b in a case shown in FIG. 24) for chromatic aberration correction for alignment light in the projection optical system are irradiated with exposure light and therefore the optical members are heated, deformed and broken. In particular, if the exposure light source is a pulse light source for eximar laser or the like, the energy density per unit time is raised excessively and therefore the probability of the breakage or the like of the optical member can be raised.
In a case where a light source such as a mercury lamp is used as the exposure light source which is continuously operated in terms of time, the probability of the breakage of the optical member for correcting the chromatic aberration can be lowered. However, the aforesaid optical member is heated due to its absorption of a portion of exposure light and the heat is transmitted to another image forming optical member, causing the other optical member to be deformed with heat or to have its refraction factor changed. As a result, a problem arises in that the image forming characteristics deteriorate. The aforesaid problem, of course, takes place in a case where the eximar laser light source is used as the exposure light source. Accordingly, use of a lighting optical system having a large .sigma. value (for example, .sigma.=0.8) which is a value indicating the coherency of illuminating light has been studied with the recent advance of the resist technology.
In order to overcome the aforesaid problem, it is necessary for the optical member for correcting the chromatic aberration to be disposed in the periphery of a optically Fourier-transformed plane with respect to the reticle pattern surface in the projection optical system under exposure light. Since the .sigma. value of the illuminating beam is 0.3 to 0.6 in an ordinary projection exposure apparatus, the distribution of irradiation light (exposure light) in the aforesaid plane is limited to the vicinity of the central portion of a circular plane and therefore it does not present in the periphery. Furthermore, although light diffracted by the reticle pattern reaches, its intensity is weak in comparison to that of rectilinear propagation light. In addition, the intensity is further weakened because diffraction light scatters in all directions due to the mixture of various pitch and direction patterns in the reticle pattern. Therefore, energy capable of breaking the optical member for correcting the chromatic aberration cannot be obtained. Furthermore, the calorific value is sufficiently limited.
However, by virtue of the improvement in the resolution and the depth of focus of the projection optical system, a technology capable of varying the shape of the irradiation light source has been reported. In an irradiation system of the aforesaid type, the light quantity distribution is not concentrated to the central portion in the plane corresponding to the Fourier-transformed plane but the same is concentrated to a specific position in the periphery. Furthermore, the concentration portion can be shifted in accordance with the reticle pattern. If the aforesaid irradiation technology is put into practical use, the position of the exposure beam which passes through the projection optical system is also changed in accordance with the change in the shape of the irradiation light source. Therefore, an improvement must be so realized that, even if the position of the irradiation light source is changed, the optical member for correcting the chromatic aberration in the projection optical system is not irradiated with irradiation light (exposure light).
Incidentally, the aforesaid technology has been disclosed in, for example, SPIE 1974-63 "New Imaging Technique for 64M DRAM" published in 1992.
Incidentally, since it is difficult for the alignment optical system which uses irradiation light having the wavelength in a region which is the same as that of exposure light to simultaneously perform the exposure operation and the alignment position at each exposure step, the alignment by using alignment light is performed prior to starting the exposure.
The reason for this lies in that the irradiation light quantity of the projection optical system cannot be considerably reduced in a case where a consideration is made about a system for, by using a half mirror, branching irradiation light (KrF eximar laser or the like) emitted from one irradiation light source into the irradiation system (hereinafter called a "main irradiation system) and the alignment optical system of the projection optical system, and therefore the branching ratio realized by the half mirror must be mainly distributed to the main irradiation system causing the light quantity of the alignment optical system to become too small if the exposure operation and the alignment operation are performed simultaneously by using the same light source.
In a case where a portion of irradiation light with which the reticle pattern is irradiated is shielded by the alignment optical system, the alignment optical system or a portion of the same must be moved outside the irradiation region at the time of exposing the pattern to light. Therefore, the position deviation between the reticle and the wafer cannot, of course, be detected during the exposure process.
Incidentally, it is ordinarily arranged that the position of the stage (the wafer stage), on which the wafer (the substrate) is placed, is detected by a laser interference meter at an excellent accuracy by means of a feedback control method which uses a closed loop. However, there arises a problem in that fluctuation is superposed on the detection signal obtained by the laser interference meter due to change in the refraction factor of air due to fluctuation. As a result, even if the wafer stage is located to a position, fluctuation takes place relative to the aforesaid point due to the fluctuation.
The amplitude of the fluctuation becomes larger in inverse proportion to the frequency and the fluctuation actually takes place in a range about .+-.0.03 .mu.m while taking the response of the stage into consideration. Hence, a problem takes place in that even if the reticle and the wafer are correctly aligned to each other prior to starting the exposure process, the accuracy deterioration cannot be prevented if the alignment operation is not always performed during the exposure process.
If irradiation light having a wavelength which is different from that of exposure light is used on the contrary, the system capable of performing the alignment operation during the exposure operation can be constituted as described above. However, various limits present in the optical system for correcting the chromatic aberration and disposed between the reticle and the wafer causes a problem to take place in that the chromatic aberration (the chromatic aberration on the axis and the chromatic aberration of the magnification) cannot be completely corrected over the all beams and therefore the offset error can easily be generated.